This invention relates to a process for purifying a halongenated aromatic compound, particularly to a process for purification of a brominated aromatic compound, such as decabromodiphenyl ether by recrystallization.
It is known that in the reaction of bromine with diphenyl ether in the presence of anhydrous aluminum chloride, the main product, decabromodiphenyl ether (DBDPE), was obtained in the form of a brown-colored precipitate together with hydrogen bromide, bromine, and some lower brominated diphenyl ethers. The DEDPE may be separated from the reaction products to a great extent by neutralizing with aqueous NaOH, washing with water, and then filtering. The solid obtained was dried at 100 degrees C. and 10 mm Hg for two hours to give solvent-free DBDPE of 10-200.mu. particle size.
Although the process mentioned above could remove most of the impurities, the small amount of hydrogen bromide which was wrapped in the precipitates would cause the DBDPE to turn to a brown color when heated in the oven over along period of time. Many other proceses to purify the DBDPE from bromine contaminations by grinding it to small particle sizes and heating to a high temperature have been proposed. Examples of these processes are disclosed in U.S. Pat. No. 4,327,227, UK Patent Application GB No. 2,081,253, Japanese Patent Publication Nos. 78,116,332 and 78,116,334, French Patent Application FR No. 2,487,821, Japanese Publication No. 78 53,629, Japanese Publication No. 78, 21,129, and Ger. Offen. No. 2,984,149.
U.S. Pat. No. 4,327,227 discloses a process in which the thermally stable crude decabromodiphenyl ether is ground to reduce particles predominantly less than about 20 microns in diameter and thereafter heated at a temperature about 150.degree.-300.degree. C. for a time sufficient to effect substantial removal of the impurities therefrom. It is also disclosed therein that decabromodiphenyl ether is so insoluble in known solvents that purification by recrystallization is uneconomical and impractical.
U.S. Pat. No. 3,673,262 discloses a crystallization process for purification of gemdiphenylol alkanes and more especially the production of 2,2-bis (4-hydroxyphenyl) propane or bisphenol-A of a very high purity. The crystallization is effected by dissolving the above compound in a solvent and heating the resultant material to a temperature at least 5.degree. C. above the atmospheric boiling point and at a pressure above the vapor pressure of the resultant material at that temperature. The process enables some difficulty soluble compounds to be dissolved efficiently in some solvents.
While the process disclosed in U.S. Pat. No. 3,673,262 is efficient, it is practically unsuitable for decabromodiphenyl ether, with impurities such as bromine and hydrogen bromide, since the decabromodiphenyl ether with these impurities is corrosive, especially when it is heated to an elevated temperature.